The present invention relates to a gas diffusing electrode body that can be used, for example, for manufacturing a fuel cell, to a method of manufacturing such an electrode body and also to an electrochemical device.
In recent years, there has been an ever-increasing strong demand for alternative clean energy sources that can replace fossil fuel including petroleum. Hydrogen and hydrogen gas fuel are attracting attention as such energy sources.
It is believed that hydrogen is an ideal energy source because it contains a large amount of chemical energy per unit mass and emits neither harmful substances nor gas that warms the earth. Hydrogen is a clean, ubiquitous and inexhaustible energy source.
Recently, efforts have been paid to develop fuel cells adapted to draw electric energy out of hydrogen energy. Expected applications of such fuel cells include large scale power plants, on-site home power generators and power sources of electric automobiles.
A fuel cell is formed by arranging fuel electrodes, which may typically be a hydrogen electrode and an oxygen electrode, oppositely with a proton conductor film sandwiched between them. A cell reaction occurs to give rise to electromotive power as hydrogen and oxygen are supplied to the respective electrodes as fuel. When manufacturing a fuel cell, a proton conductor film and fuel electrodes, which may typically include a hydrogen electrode and an oxygen electrode, are molded separately and subsequently bonded together.
The fuel electrodes, or gas diffusing electrodes, of a fuel cell, which may typically include a hydrogen electrode and an oxygen electrode, are mainly made of electro-conductive carbon particles and have a catalyst layer that carries catalyst metal such as platinum.
Conventionally, a gas diffusing electrode is manufactured either by molding catalyst particles, which are powdery or granular electro-conductive carbon particles carrying platinum as catalyst, into a sheet along with water-repelling resin such as fluorine resin and an ion conducting material or by using a step of directly applying the catalyst and the other ingredients onto a carbon sheet. Japanese Patent Application Laid-Open Publication No. 5-36418 discloses a gas diffusing electrode to be used for a solid polymer fuel cell, which is manufactured by applying powdery or granular carbon carrying platinum onto a carbon sheet along with water-repelling resin and an ion conducting material.
A gas diffusing electrode refers to an electrode having continuous pores through which working gas can be diffused. A gas diffusing electrode also shows an electron conducting property.
When a gas diffusing electrode is used as hydrogen decomposing electrode of a fuel cell that may be of a solid polymer type, fuel is ionized by the catalyst of platinum and electrons produced by ionization flows through the powdery or granular electro-conductive carbon, while protons (H+s) produced as a result of ionization of hydrogen flows to the ion (proton) conducting film by way of ion conductors. This process requires gaps allowing gas to pass through, electro-conductive powdery or granular carbon, ion conductors and a catalyst for ionizing fuel and/or an oxidizing agent. Fuel is ionized by the catalyst which is typically platinum and electrons produced as a result of ionization flows through the electro-conductive powdery or granular carbon, while ionized hydrogen (protons) flows to the ion conducting film by way of the ion conducting material. This process requires gaps allowing gas to pass through, electro-conductive powdery or granular carbon, ion conductors and a catalyst for ionizing fuel and/or an oxidizing agent. If fuel is hydrogen, a reaction ofH2→2H++2e−
takes place in the gas diffusing electrode (catalyst layer) of the fuel cell, while a reaction ofO2+4H++4e−→2H2O
occurs in the oxygen electrode to produce water.
There is an increasing demand for fuel cells of the type under consideration that can generate a large amount of power per unit volume. However, the use of a large gas diffusing electrode is required to increase the rate of power generation. On the other hand, there is an urgent demand for fuel cells comprising a low profile gas diffusing electrode.
When the catalyst substance, which may typically be platinum, contained in the gas diffusing electrode is mixed with the other ingredients in a powdery or granular state, the area in which it is brought into contact with protons (H+s) can be insufficient. In other words, the electrode reaction can be insufficient.